1. Field of the Invention
The present invention relates to a photonic sampler device used in a system for analyzing optical or electric transients.
2. Description of the Prior Art
The recording of rapid transients over a large number of samples raises problems for sampling rates less than a nanosecond if it is intended to use only solid state circuits. Moreover, in this high frequency range a greater and greater percentage of signals are conveyed by optical fibers which signals therefore must be translated into electric signals, prior to recording, in receivers which are band limited.
The slit scanning camera, known as a streak camera is used for recording multiple optical signals whose resolution in time is less than a hundred or so picoseconds. The phenomenon to be observed, whether it is electrical or optical, is applied to an input interface circuit which facilitates calibrated optical signals. These optical signals are focused by a lens on a slit, the image of this slit being projected on the photocathode of the image converter tube by means of a take up lens. The optical signals may also be translated through a fiber bundle which ends on the camera side in a light slit represented by linearly juxtaposed fibers, and then are applied against the input window of an image converter tube. The streak camera comprises at the input a photocathode and at the output an anode formed by a fluorescent screen, and between the two grids are deflecting electrodes. Considering a number n of channels each corresponding to a fiber of the slit, at the input, n juxtaposed optical channels are available in a line and which, by deflection, will be moved vertically so as to display for each of these n channels the variation in time of the corresponding phenomenon.
At the output of the streak camera, the signal may be applied to a television camera if the gain is sufficient, this may be the case with a slit camera equipped with an image converter tube with incorporated microchannel pancake. If not, there is generally disposed between the streak camera and the television camera an image intensifying tube for increasing the light intensity of the signal and re-establishing a sufficient gain. The fluorescent screen of the streak camera is coupled optically to the input photocathode of the intensifier tube, such coupling being generally assumed by an optical fiber pancake. Similarly the output of the intensifier comprises a fluorescent screen, which is coupled by a fiber pancake to the television camera. This is preferably of solid state circuit type, such as a charge transfer device. It comprises an X, Y matrix of photosensitive elements which receive the light flux of the image at the output of the intensifier and this matrix is itself followed by preamplification and reading circuits for producing a video signal similar to that of television line by line scanning.
The advantage of the streak camera as an optical transient recorder is due to the substantially instantaneous transformation of the light into electrons at the photocathode and to the storage of the signal at the level of the screen, which allows it to be taken up by a video camera and to be digitized in deferred time. Another advantage is that amplification of the signal may be achieved with a very wide pass band.
However, systems using streak cameras capable of recording up to 50 channels generally, have a certain number of drawbacks likely to limit the fields of use thereof.
A first aspect to consider is that of reliability. It will be readily understood that if the streak camera breaks down, all the channels break down and it may be unacceptable to contemplate even a very low probability of losing the end measurements for a breakdown which affects this element of the chain.
If we now consider the dynamic aspects in time and in amplitude, the tube of the streak camera is designed for recording images and therefore it comprises relatively large photocathode and fluorescent screen surfaces which require fairly complex electronic optics while only allowing average spatial dynamics. Such dynamics are further reduced if the image is automatically taken up by means, for example, of a charge transfer device whose dimensions are small with respect to those of the fluorescent screen. The resolution of solid state cameras is limited at the present time to about 500.times.500 points and the possibility of recording in channels of 150.times.200 samples seems to be a maximum. It should be noted that the same analysis time is required for the different input channels and that all the channels are initiated at the same time.
In addition, the juxtaposition of the input channels causes the creation of cross talk defects corresponding to the overflow of light from one channel to the neighboring channel which further limits the response dynamics of the system. Finally, it should be noted that the defects of the chain include those of the complementary elements required between the streak camera and the solid state camera which are the light intensifier and a matching optical means between the fluorescent screen at the output of the intensifier and the solid state camera, this optical matcher being formed for example by an optical cone.
Another drawback results from the fact that the system with a streak camera requires, downstream of the solid state camera, the use of a relatively complex image memory for its operation.